Adds support for using dockers memory limiter.

The default is nil, and will default to dockers own defaults. It
must be set in bytes.
Merge pull request #9 from gregf/memory_limit

Adds support for using dockers memory limiter.
Add options for cpu shares, custom dns, and data volumes.
Merge pull request #11 from gregf/more_options

Add options for cpu shares, custom dns, and data volumes.